Little improvement in the dental tooth bur art has been made in the past 90 years. The tooth burs currently in use reflect only slight modifications to those described in U.S. Pat. No. 418,108 (Browne), dated 1889. Browne describes two versions of a dental tooth bur, one having straight flutes, and one having crosscut flutes. Both types of flutes have distinct limitations.
When a tooth cavity is prepared for filling with a dental bur having smooth flutes (ones without crossgrooves), a large amount of heat is generated, the enamel is not cut efficiently, and the cavity walls do not have any interiorly-projecting microretentive ridges to assist in the retention of the filling material. However, a cavity prepared with a noncrosscut bur does have the advantage of a completely straight wall, which provides the greatest shear strength against fracture due to mastication pressure because the enamel is cut completely parallel to the origin of the enamel rod crystals and as near perpendicular to the surface of the tooth as possible. This shear strength is especially important at the upper edge of the wall because of the exposed margin or interface between the filling material and the tooth enamel. It is this margin area which is especially susceptable to microfracturing.
By contrast, when a tooth cavity is prepared for filling with a bur having crossgrooved flutes, the cutting can be done more efficiently and with less heat. The cutting also produces generally annular microretentive ridges to assist in the retention of the filling material. However, a cavity prepared with a crossgrooved bur has the serious deficiency of having the microretentive ridges near the upper edge of the cavity wall. As a result, normal mastication pressure can eventually cause microfracturing of the tooth enamel and the filling material at the margin of the enamel and the filling material. This microfracturing produces small voids in the margin area. These voids contribute to corrosion of the filling material, plaque retention, percolation of fluids within the filling, enamel decalcification, and other forms of tooth deterioration.
The problem is presented, therefore, of how to obtain the beneficial aspects of a cavity prepared with a crossgrooved bur and yet minimizing microfracturing, as would be possible with a bur without crossgrooves. Obviously, a dentist could first cut and remove the decayed portion of the tooth with a crossgrooved fluted bur and then change to a smooth (or noncrossgrooved) fluted bur to redefine the upper portion of the cavity. This process would allow the efficient, low-heat cutting of the enamel to prepare the tooth and would produce microretentive ridges in the lower portion of the prepared cavity walls, while leaving the margin areas smooth and honed to allow the enamel and filling material to have maximum strength. As this method would present a complication and an addition to the normal procedures followed for preparing cavities for filling, it is not a completely satisfactory solution to the problem.
A more practical solution would be to provide a tooth bur capable of both the honing of a cavity wall and the production of ridges in that wall. However, in the approximately 90 years since the issuing of the Browne patent, no such improvement has been achieved.
Another problem with the tooth burs found in the prior art is that such burs generally have right-angled edges between the portions of the bur which cut the cavity wall, and the portions of the bur which cut the cavity floor. Such burs produce a cavity preparation having a cavity wall which intersects the cavity floor at approximately a right angle, thereby producing sharp internal line angles in the tooth enamel and/or dentin at this point. These line angles are internal stress points for possible fracturing of the restored tooth. If the sharp internal line angles of the cavity preparation could be eliminated, the completed preparation would be much stronger.
Accordingly, it is the principal object of the present invention to provide a dental bur capable of simultaneously smoothly honing the upper portion of a cavity wall and creating internally-projecting microretentive ridges in the lower portion of the cavity wall.
It is a further object of this invention to simultaneously hone the upper portion of a cavity wall and create microretentive ridges in the lower portion of the cavity wall with dental burs of different shapes.
It is another object of the present invention to prevent microfracturing at the upper portions of a tooth cavity wall.
Yet another object of the present invention is to prepare a tooth cavity wall quickly and efficiently with a minimum of heat generation.
Another object of the present invention is to provide a sufficient number of generally annular microretentive ridges in the lower portion of a cavity wall to assist in the retention of the material used to fill the cavity without weakening the upper portion of the cavity wall, thereby eliminating or minimizing the possibility of microfracturing of the tooth and/or filling material.
A further object of the present invention is to reduce the sharp internal line angles in a cavity preparation to strengthen the finished restoration of the tooth by reducing the possibility of tooth fracture at the junction of the cavity wall and floor.